1. Field of the Invention
The invention relates generally to the field of apparatus and methods for measuring drilling parameters and/or formation parameters during wellbore drilling. More particularly, the invention relates to structures and techniques for making such measurements in drilling operations known as “casing drilling.”
2. Background Art
Wellbores are drilled into the Earth's subsurface to recover hydrocarbons and other desirable materials trapped in geological formations in the subsurface. A wellbore is typically drilled by advancing a drill bit through the subsurface formations. The drill bit is attached to the lower end of a “drill string” suspended from a drilling rig. The drill string is a long string of sections of drill pipe that are connected together end-to-end to form a long shaft for driving the drill bit further into the subsurface. A bottom hole assembly (“BHA”) containing various instrumentation and/or devices to control the mechanical properties of the drill string is typically provided above the drill bit. Drilling fluid, or “mud”, is typically pumped down through the drill string to the drill bit. The drilling fluid lubricates and cools the drill bit, and it carries drill cuttings back to the surface in the annulus between the drill string and the wellbore wall.
In conventional drilling, a well is drilled to a selected depth, and then the wellbore is typically lined with a larger-diameter pipe, usually called “casing.” Casing typically consists of casing sections connected end-to-end, similar to the way drill pipe is connected. To accomplish this, the drill string and the drill bit are removed from the borehole in a process called “tripping.” Once the drill string and bit are removed, the casing is lowered into the well and cemented in place. The casing protects the wellbore wall from collapse and isolates the subterranean formations from each other. After the casing is in place, drilling may continue by lowering a drill bit through the casing and continuing to drill once the drill bit reaches the bottom of the well.
Conventional drilling typically includes a series of drilling, tripping (removing the drill string from the wellbore), cementing in place a pipe or casing to protect the drilled wellbore, and then resuming drilling using a smaller diameter drill bit to extend the wellbore. The foregoing process is very time consuming and costly. Additionally, other problems may be encountered when tripping the drill string. For example, the drill string may get caught up in the borehole while it is being removed. These problems require additional time and expense to correct.
The term “casing drilling” refers to the use of a casing string in place of a drill string. Like drill string, a string of casing sections are connected end-to-end to form a casing string. The BHA and the drill bit are connected to the lower end of a casing string, and the well is drilled using the casing string to transmit drilling fluid, as well as axial and rotational forces, to the drill bit. Upon completion of drilling, the casing string may then be cemented in place to from the casing for the wellbore. Casing drilling enables the well to be simultaneously drilled and cased, thus eliminating one of the tripping steps necessary in conventional drilling.
One technique for casing drilling is described in U.S. Pat. No. 7,004,263 issued to Moriarty, et al., and assigned to the assignee of the present invention. Another casing drilling technique is described in U.S. Pat. No. 6,705,413 issued to Tessari which includes a retrievable drill bit mounted at an end of the casing string and either a mud motor with a bent housing and/or bent sub or a rotary steerable tool used to direct the bit to drill directionally.
The foregoing techniques require expensive, difficult to operate drill bits that can be retrieved through the casing after the intended wellbore depth is reached in order to provide any form of measurement or directional control, such as using measurement while drilling (“MWD”) and logging while drilling (“LWD”) devices. Such bits must drill a larger diameter hole than the casing, and then be reduced in diameter so that the BHA can be retrieved to the surface through the casing after the intended wellbore depth is reached.
What is needed is a system to enable casing drilling that can eliminate the need for drilling components that are retrievable through the casing.